The first thing i'd say is that its pretty sunny there, generally speaking. 
If your outdoors during the day, chances are low you'll need a flash at all. 
In June normally all Thailand except the southern Gulf is in monsoon. The southern Gulf's (Samui and neighbours plus adjacent mainland coast) usual wet season is Oct into Jan. I can't speak for what the day light is like during that time however.

The recommended range for the flash isn't more than about 100mm, past that, its usefulness is limited. Using the integrated diffusion panel allows extended wide-angle coverage to 14mm.

It wouldn't hurt to have a better flash than the integrated one but it all depends on what you'll be shooting. With the 15-55mm lens i'd expect the integrated flash to do good enough.

It really depends on what you'll be shooting and what you'll be using the flash for. I used a couple of generic hand-held flashes for light painting in caves in Thailand. I thoroughly recommend them for this purpose! 


Check out the DPreview link [here][1] for details about the flash and base your recommendations around that.


Hope that helps!

PS: Interstingly, I just came back from Thailand. All my camera gear got destroyed cause my peli-case leaked -_- Watch out for the water!
  [1]: http://www.dpreview.com/news/2005/8/22/canon430ex